The present invention relates to a color picture tube mask frame fitted on a molded shadow mask.
FIG. 1 is a plan view showing a shadow mask plate prior to press molding. A shadow mask plate 7a made of aluminum killed steel has notches 8 at its corners to prevent a skirt portion 4 from being longer at the corners than at other portions since a plate material tends to concentrate on corners during press molding. An apertured portion 5a has a large number of circular or slit-like apertures, and a nonapertured portion 5b as a peripheral portion surrounds the apertured portion 5a. Notches 4a are used to position the shadow mask plate in press molding.
FIGS. 2(a) and 2(b) show a rectangular shadow mask after press molding, in which FIG. 2(a) is a perspective view thereof and FIG. 2(b) is a sectional view thereof taken along the line IIb--IIb of FIG. 2(a). Referring to FIGS. 2(a) and 2(b), reference numeral 4 denotes a skirt portion; 5C, a spherical bead; 6, a corner thick wall portion formed at a notch 8; and 7, a molded shadow mask.
In general, since the molded shadow mask 7 is obtained by press-molding a relatively narrow area as the nonapertured portion 5b of a plate having a relatively small thickness (e.g., 0.13 mm), press moldability is improved by forming the corner notches 8. When each corner notch 8 is too large to form a gap at the corresponding corner after press molding to obtain the shadow mask 7, light leaks from the gap during the BM exposure as the postprocess, and the corner portion having the gap is exposed. Graphite is then not attached to the exposed corner portion. As a result, when the shadow mask 7 is assembled as a picture tube, the picture tube becomes defective because the non-blackened corner portion becomes a so-called film defect.
In order to prevent the above drawback, the thick wall portion 6 shown in FIG. 2(b) is formed at the corner notch 8, thereby preventing formation of a gap in the corner.
FIG. 3 is a perspective view showing a conventional mask frame 9. A fitting portion between the mask frame 9 and the molded shadow mask 7 has a size slightly smaller than a size of a punch of a shadow mask mold to assure a fitting margin. The outer appearance of the mask frame is similar to that of the punch. The mask frame 9 has a thickness larger than that of the molded shadow mask 7 and is subjected to perfect drawing. Therefore, the mask frame 9 does not require thick wall portions 6, unlike the shadow mask 7.
With the above structure, when press molding is performed for a long period of time, the punch of the mold is worn out to gradually increase the size of the thick wall portion 6. For this reason, when the shadow mask 7 shown in FIG. 2(a) is fitted in the mask frame 9 shown in FIG. 3, an amount LA of spread in the molded shadow mask 7 by the thick wall portions 6 is increased, as shown in FIG. 4, thereby undesirably causing mask deformation.
When the molded shadow mask 7 is connected to the mask frame 9 by a spot welding machine 10, stress acts on the spherical surface of the apertured portion 5a to distort the spherical surface. For this reason, when the molded shadow mask 7 is assembled as a picture tube, phosphor arrays of the picture tube are disturbed to cause so-called grouping, thereby greatly degrading the landing characteristics.